Liquid crystal displays are used in a wide variety of commercial applications including portable (laptop) computers, wristwatches, camcorders and large screen televisions. Liquid crystal light valves, used as spatial light modulators, may be used in projection systems as well as optical computing applications. Limitations inherent in the existing technology come from the necessity of fabricating the displays on transparent glass or quartz substrates which are not amenable to high quality electronic materials. Fabrication of displays on bulk silicon, although of high crystal quality, unnecessarily constrains the display to reflective mode schemes due to the opaque substrate and is not applicable to transmissive applications. The ability to integrate drive circuitry using thin-film transistors (TFTs) with liquid crystal displays has improved reliability and has allowed the use of this technology in lightweight, portable applications. However, the integration of display driving circuitry heretofore has been substantially limited to thin film transistor technology using amorphous (a-Si) or polycrystalline (p-Si) silicon deposited on the glass or quartz substrate. The intrinsic properties such as lattice and thermal mismatch between the silicon layer and the substrate, and the low temperature deposition techniques used in the a-Si and p-Si technologies result in a silicon layer with poor charge carrier mobility and crystallographic defects. These limitations are directly related to inferior electronic device performance and limitations when compared to bulk silicon.
Of particular importance for integrated display systems is the desire for higher density circuitry for ultra-high resolution display and light valve applications and for the monolithic integration of display driver circuitry and related signal processing circuitry on-chip. The characteristic lower (electrical and crystallographic) qualities of a-Si and p-Si materials result in poor fabrication yields when compared to conventional Very Large Scale Integration (VLSI) processing. Overcoming this problem, inherent in the poorer quality amorphous or polycrystalline material, requires the use of redundant circuit elements in each pixel to ensure fully functional displays in a-Si and p-Si. This redundancy requires an concomitant increase in the picture element (pixel) size thereby inhibiting the ability to scale displays and light valves to ultra-high resolution. The additional circuit elements also reduce the aperture ratio, i.e. the fraction of pixel area allowing transmitted light, thereby reducing the brightness of the display or light valve.
Furthermore, the low carrier mobility, low speed, low yield a-Si and p-Si materials are incompatible with VLSI design and fabrication techniques which would otherwise readily allow integration of video drivers, digital logic and other computational circuitry on-chip thereby offering designers greater functionality, higher reliability, and improved performance. The prior art has attempted to overcome the material problems associated with a-Si and p-Si using a novel crystalline silicon process to implement drive circuitry on the display. U.S. Pat. No. 5,206,749 entitled xe2x80x9cLiquid Crystal Display Having Essentially Single Crystal Transistors Pixels and Driving Circuitsxe2x80x9d by P. M. Zavracky et al. teaches an approach where the electronics are fabricated on an opaque recrystallized silicon layer. This silicon-on-insulator material is prepared by a so-called isolated silicon epitaxy (ISE) process. The display circuitry is then lifted off and transferred to a transparent substrate.
B. Bahadur, editor, Liquid Crystals: Applications and Uses, Vol. 1, World Scientific, New Jersey, 1990, pp. 448-451 reviews the state of the art in active matrix displays for projection display applications. Active-matrix displays use one or more nonlinear circuit elements, e.g. TFTs or diodes, to switch the liquid crystal capacitor in each pixel. Among the materials discussed for these applications included silicon-on-sapphire (SOS). The authors state recognized limitations of SOS on page 450 xe2x80x9calthough SOS devices have excellent performance in terms of drive current and speed, they have leakage currents which are too high for use in an active matrix display.xe2x80x9d These limitations are further evidenced by table 16.3 and FIG. 16.9 on page 451 showing excessive leakage currents for SOS TFT devices. The excessive leakage results in a drop in voltage across the liquid crystal capacitor which, in the case of commonly used nematic liquid crystals, results in a orientational change and change in gray level. These known and recognized limitations of SOS are evidence that it is contrary to what is allegedly known in the art and therefore it is not obvious to one skilled in the art that SOS could be used for an active matrix display using nematic liquid crystals without introducing inferior changes of light levels in the display.
Thus, in accordance with this inventive concept a continuing need has been recognized in the state of the art for an electrically addressable ultra-high resolution nematic liquid crystal display or light valve system which monolithically integrates an active matrix display with its associated drive and image processing circuitry and an apparatus and method of fabrication of an electrically addressable ultra-high resolution crystal display which may include nematic or ferroelectric liquid crystal capacitors formed on an ultra-thin silicon-on-sapphire structure (UTSOS) which allows VLSI fabrication techniques, has a transparent substrate, and uses high performance, low leakage circuit elements (MOSFETs) that allow monolithic fabrication of a complete display or light valve system.
The present invention provides a method for fabricating a monolithically integrated liquid crystal array display and control circuitry on a silicon-on-sapphire structure comprises the steps of: a) forming an epitaxial silicon layer on a sapphire substrate to create a silicon-on-sapphire structure; b) ion implanting the epitaxial silicon layer; c) annealing the silicon-on sapphire structure; d) oxidizing the epitaxial silicon layer to form a silicon dioxide layer from a portion of the epitaxial silicon layer so that a thinned epitaxial silicon layer remains; e) removing the silicon dioxide layer to expose the thinned epitaxial silicon layer; f) fabricating an array of pixels from the thinned epitaxial silicon layer wherein each of the pixels includes a liquid crystal capacitor; and g) fabricating integrated circuitry from the thinned epitaxial silicon layer which is operably coupled to modulate the pixels. The thinned epitaxial silicon supports the fabrication of device quality circuitry on the silicon-on-sapphire structure which is used to control the operation of the pixels. The types of liquid crystal capacitors that may be fabricated in conjunction with the invention may be ferroelectric liquid crystal capacitors and nematic liquid crystal capacitors.
The present invention also provides a liquid crystal display which includes a silicon-on-sapphire structure. The silicon-on-sapphire structure includes an epitaxial silicon layer formed by oxidizing the epitaxial silicon layer to form a silicon dioxide layer from a portion of the epitaxial silicon layer. The silicon dioxide layer is removed from the epitaxial silicon layer not consumed by the oxide so that a thinned epitaxial silicon layer remains. The invention further includes an array of liquid crystal capacitors formed on the thinned silicon-on-sapphire structure and integrated circuitry operably coupled to modulate the liquid crystal capacitors. The integrated circuitry is formed from the thinned epitaxial silicon layer. The thinned epitaxial silicon layer is an important feature which makes possible fabrication of device quality circuitry on the silicon-on-sapphire structure.
Other advantages of the invention will become more apparent upon review of the following specification, including the claims, and the accompanying figures.